I do not know about an action replay code for Pokemon Diamond
involving shiny pokemon, and if there was one, which I highly think
so, there would be a code at "www.codejunkies.com". Other than
that, oh well!

What you are saying is the "Permanent" shiny Pokemon code, which
is not available,
but the code for temporary code is available. Go onto a website
to look for codes like
the "shiny" code or onto Youtube.